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Balancing the stiffness and toughness is always the goal of high performance general plastics, especially for isotactic polypropylene/graphene oxide (iPP/GO) composites. However, during the crystallization of β‐nucleated iPP, GO exhibits an α‐nucleating effect, and its nucleation efficiency (NE) is higher than that of some conventional β‐nucleating agents (β‐NAs), thus reducing the formation of β‐phase and therefore the toughness of the material. In this article, we used the melt memory effect of β‐nucleated iPP/GO composites to improve the formation of β‐phase. By adjusting the fusion temperature Tf of β‐nucleated iPP/GO composites, the amount of ordered structure in composites was controlled. The crystallization behavior and melting behavior of composites were investigated by differential scanning calorimetry (DSC) and wide‐angle X‐ray scattering. The results showed that when Tf = 200°C, no ordered structures survived in the melt, no β‐phase formed due to the strong α‐nucleating effect of GO; in the Tf range of 178–168°C, large amount of β‐phase survived in the composites, which helped to improve the formation of β‐phase during the subsequent cooling process, and the relative content of β‐phase even reached the maximum value of 85.8%. The mechanism of melt structure on the polymorphic behavior of the composites were also investigated, and it was found that the ordered structure in the melt evidently enhanced the NE of β‐crystallization, and overcome the challenge from the α‐nucleation effect of GO. POLYM. COMPOS., 40:E440–E448, 2019. © 2018 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]

Abstract: The effects of ordered structure on isothermal crystallization kinetics and subsequent melting behavior of β‐nucleated isotactic polypropylene/graphene oxide (iPP/GO) composites were studied using differential scanning calorimetry. The ordered structure status was controlled by tuning the fusion temperature (Tf). The results showed that depending on the variation of crystallization rate, the whole Tf range could be divided into three regions: Region I (Tf > 179 °C), Region II (170 °C ≤ Tf ≤ 179 °C) and Region III (Tf < 170 °C). As Tf decreased from Region I to Region III, the crystallization rate would increase substantially at two transition points, due to the variation of the ordered structure status. Calculation of Avrami exponent n indicated that the ordered structure induced the formation of two‐dimensional growing crystallites rather than three‐dimensional growing crystallites. Moreover, in the case of isothermal crystallization, the ordered structure effect (OSE) can also greatly increase the relative content of β‐phase (βc). In Region II, OSE took place, resulting in evident increase of βc, achieving 92.4% at maximum. The variation of the isothermal crystallization temperature (Tiso) had little influence on the Tf range (Region II) of the OSE. The higher Tf in Region II was more favorable for the formation of higher βc. The ordered structure was favorable for the improvement of the nucleating efficiency of β‐nucleating agent (β‐NE), and was more effective for the improvement of lower β‐NE. © 2018 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

In this study, polyamide-aluminum foil-polypropylene (PA-Al-PP) composite films with different orientation status of the PP layer were prepared, and their morphology, tensile, peeling and heat seal behavior were studied. The comparative study of tensile and fracture behaviors of single-layer film of PA, Al and PP, as well as the composite films of PA-Al, PP-Al and PA-Al-PP revealed that in PA-Al-PP composite film, the PA layer with the highest tensile strength can share the tensile stress from the Al layer during stretching, while the PP layer with the lowest tensile strength can prevent further development of the small cracks on boundary of the Al layer during stretching. Moreover, the study of heat seal behavior suggested that both the orientation status and the heat seal conditions were important factors in determining the heat seal strength (HSS) and failure behavior of the sample. Four failure types were observed, and a clear correspondence between HSS and failure types was found. The results also elucidated that for the composite film, only in the cases where the tensile stress was efficiently released by each layer during HSS measurement could the composite film exhibit desired high HSS that was even higher than its tensile strength. [ABSTRACT FROM AUTHOR]

To investigate effects of molecular structures on β-crystallization of polypropylene random copolymer (PPR), PPR-A and PPR-B with similar molecular weight and distribution but significantly different polymorphic behavior were used. Wide-angle X-ray diffraction (WAXD), Fourier transform infrared spectroscopy (FT-IR), solvent fractionation combined with successive self-nucleation and annealing (SSA) and Gel Permeation Chromatography (GPC) were applied. Results revealed that after addition of same amount of β-nucleating agent (β-NA), large amount of β-phase is obtained in PPR-A ( K = 0.603), while no β-phase can be seen from PPR-B; Structure characterizations revealed that they have similar molecular weight but different total amount and distribution of ethylene comonomer: the total content of ethylene comonomer of PPR-A is higher compared with PPR-B, but its ethylene comonomer distribution is less uniform. In this way, PPR-A possesses high molecular weight high isotactic fraction, which cannot be seen from PPR-B due to its more uniform comonomer distribution. Therefore, the presence of highly isotactic PP (HPP) is the key factor in β-crystallization of PPR. For verification, HPP with low or high molecular weights are respectively added into β-nucleated PPR-B. Results revealed that HPP was effective in enhancing β-crystallization of PPR-B; higher molecular weight of HPP was more favorable and more effective. [ABSTRACT FROM AUTHOR]

The isotactic polypropylene (iPP) with different uniformities of stereo-defect distribution was synthesized, and the β-nucleated iPP/glass fiber composites (TPPA-GF and TPPB-GF) were prepared. The influence of glass fiber on the non-isothermal crystallization kinetics, β-phase crystallization, and morphology behavior of the composites were investigated by means of differential scanning calorimetry (DSC), scanning electronic microscopy (SEM), and wide angle X-ray diffraction (WAXD). The results revealed that for both TPPA-GF and TPPB-GF, as the amount of GF increases, the crystallization peak temperature and relative degree of crystallinity increase gradually, while crystallization peak width decreases gradually. Meanwhile, the half crystallization time t1/2of both TPPA and TPPB decreases gradually, indicating that the addition of GF can accelerate the crystallization of both TPP-A and TPP-B. Moreover, the addition of glass fiber decreases the energy barriers ΔE and makes the composites to crystallize easier. The results of melting behavior study and WAXD indicate that high cooling rate leads to relatively low crystallization temperature and low thermal stability, resulting in higher β-phase content with relatively low thermal stability, while low cooling rate leads to high crystallization temperature and high thermal stability, which is more favorable for the formation of less amount of β-phase with high thermal stability. For both TPPA and TPPB, the higher the content of glass fiber, the lower the βc. The addition of glass fiber not only reduces the content of β-phase but also destroys its spherulite morphology. It was also observed that the glass fiber is more uniformly dispersed in TPPB compared with TPPA. [ABSTRACT FROM AUTHOR]